In lithium ion cells, the negative electrode, which is also referred to as anode, usually comprises graphite, known as intercalation graphite which is able to electrochemically incorporate and release lithium ions (Li+) reversibly into and from its three-dimensional layer structure. This is also referred to as lithium intercalation and lithium deintercalation. However, the lithium uptake capacity of graphite is limited. When the graphite is fully lithiated, the carbon atoms of the graphite are present in a ratio to the lithium atoms of six to one. The stoichiometry is accordingly Li1C6. The specific reversible theoretical capacity of intercalation graphite is 372 mAh/g.
Apart from the limitation of the specific reversible theoretical capacity of 372 mAh/g, graphite has further disadvantages:
Since the electrical conductivity of graphite alone is not sufficient for high-power applications, the negative electrode usually additionally comprises conductive carbon black. However, conductive carbon black is very difficult to disperse in the coating composition during electrode production and also reduces the energy density of the system since conductive carbon black is a passive cell component. In addition, conventional electrodes containing conductive carbon black as electrically conductive material are more difficult to compact.
In addition, the crystallographic density of graphite can increase and decrease due to the incorporation and release of lithium atoms. As a result, an electrical connection between the conductive carbon black and a metallic contact element for contacting the negative electrode can be interrupted. This leads to a loss of cyclable capacity or to an increase in the internal resistance and the impedance of the lithium ion cell.
A further disadvantage of graphite is the formation of solid electrolyte interfaces (SEI). These solid electrolyte interfaces are formed on the graphite surface but require lithium ions which are ultimately withdrawn irreversibly. This leads to irreversible “consumption” of lithium in the first charging cycle.
The document US 2009/0117467 A1 describes a material composition which comprises a mixture of an electrochemically active material and nano sized graphene platelets for a negative electrode of a lithium ion cell. The nano sized graphene platelets are composed essentially of a plate of a graphene plane or of multiplates of stacked graphene planes joined to one another by means of van der Waals forces. The document US 2009/0117467 A1 discloses that both the electrochemically active material and the nano sized graphene platelets can absorb and desorb lithium ions.